1. Field of the Invention
The present invention relates generally to solar energy collection devices and, more particularly, to parabolic trough-type reflecting concentrators. Specifically, the present invention relates to structural support mechanisms for parabolic solar energy concentrating devices.
2. Description of the Prior Art
For many years, solar energy devices have used reflecting surfaces to collect and concentrate energy from the sun for conveyance to energy conversion devices of various types. Such concentrators have taken numerous different forms over the years, such as dishes, troughs, and the like. One of the more common arrangements for concentrating sunlight, and in particular on a large scale, is by the utilization of an elongated reflective trough having a parabolic cross-section that reflects incoming sun light or solar energy to a focal line positioned along the length of the parabolic reflector. To maintain a sharp focus of concentrated sun light, such parabolic trough concentrators generally rotate about a single fixed, longitudinal axis to follow the apparent motion of the sun. Parabolic trough concentrators generally rotate about an axis extending parallel to the focal line. Conventionally, the ends of each solar concentrator are generally journalled by bearings that support the concentrator and allow the concentrator to rotate. To correctly position the concentrator device, a mechanical drive system is typically utilized, and to reduce the number of drive systems that are required for a large solar concentrator installation, several parabolic trough concentrators are commonly axially connected together in a row and are rotated using one shared drive system. Examples of such parabolic reflector devices in solar energy collection systems are illustrated in U.S. Pat. Nos. 3,915,147, 4,114,594, 4,178,913 and 4,284,063. All of these patents illustrate rotating solar collection and concentration devices of the parabolic trough-type discussed above.
When several concentrators are connected together in a row, high winds may produce large torsional forces in the parabolic trough concentrators which are extenuated due to the length of the array. Unless the row of interconnected concentrators has sufficient torsional stiffness, twisting along the longitudinal axis of the concentrators can degrade their optical performance and can permanently deform them. Moreover, such twisting can produce undue stress and strain on the bearing connections between the aligned concentrator devices. Thus, to prevent excessive twisting or damage to the concentrators, they are preferably designed to possess high torsional stiffness and support.
Prior devices have generally utilized two different approaches to provide high torsional stiffness to parabolic trough concentrators. In one such approach, the solar concentrator device is stiffened by the actual construction arrangement of the trough reflector itself. U.S. Pat. No. 3,841,738 discloses one such device wherein a parabolic reflector is formed utilizing an expanded honeycomb core which separates sheets of skin material that adhere to the top and bottom surfaces of the honeycomb core, thereby increasing the thickness and torsional stiffness of the reflector device. U.S. Pat. No. 4,240,406 illustrates another parabolic reflector that is formed by utilizing rigid bulkhead members with spaced sheet metal skins which are affixed to the top and bottom surfaces of the rigid bulkhead members, the bulkhead members being aligned perpendicularly to the focal line and rotational axis of the device. Problems associated with solar concentrators constructed from these type of arrangements is that such construction provides substantially additional weight and cost of the materials which are utilized to increase thickness of the parabolic reflector.
Another common method for providing torsional stiffness to solar concentrator devices is by securing a stiff metal tube or rod or other elongated member to the back of each solar concentrator. U.S. Pat. No. 4,268,332 illustrates such a device wherein a metal tube aligned parallel with the focal line of the concentrator is incorporated within the back portion of the concentrator device itself. To provide appropriate torsional strength, the metal tube must be relatively large and heavy and thereby produces a disadvantage similar to that described above due to added weight and cost of the device.
Yet another consideration in evaluating torsional support structures for solar concentrators is the interconnection of the ends of the solar concentrators when two or more concentrators are aligned and joined for common rotational motion. In such arrangements, the concentrators are arranged in a row and joined together with the ends of the solar concentrators typically being joined utilizing stiff metal shafts. When considering torsional support, the metal shafts must have enough torsional stiffness to transfer any wind-induced twisting loads from one solar concentrator to the adjacent solar concentrator. High torsional stiffness of the metal shafting is generally obtained by increasing the diameter of the shafting, and to support the large metal shafting, large bearings are required. A disadvantage of this type of conventional construction for interconnecting solar concentrators is the high weight and cost of the metal shafting that connects adjacent solar concentrators as well as the high cost of the bearings that support the metal shafting. Thus, there is still a need for a support mechanism which provides high torsional stiffness in a solar concentrator as well as in a group of solar concentrators that are aligned in an array, which mechanism is both low in cost as well as low in weight.